The present invention relates to an extendable shaft, for example, an intermediate shaft used in a steering apparatus of an automobile or the like.
It has hitherto been widely practiced that an expandable/contractible intermediate shaft is provided midway of a steering apparatus in order to prevent a steering wheel from thrusting a driver's body up in the event of a collision accident. This expandable/contractible intermediate shaft diminishes an entire length if a front portion of the automobile is crashed subsequent to a so-called primary impact in which the automobile collides with other automobile. Such an expandable/contractible intermediate shaft is constructed by making a serration or spline engagement between one end of an outer tube member and one end of a solid inner shaft member.
FIG. 1 shows one example of an automobile steering apparatus incorporating an intermediate shaft according to the present invention. A steering apparatus for an automobile involves the use of a mechanism, as schematically shown in FIG. 1, for transmitting a motion of a steering wheel to a steering gear. Referring to FIG. 1, a steering wheel 102 is fixed to a steering shaft 101 rotatably inserted through within a steering column 103. The steering column 103 is fixed to a schematically shown car body 106 through members 104, 105. Rotations of the steering shaft 101 are transmitted to a shaft 111 of an unillustrated steering gear portion via a steering intermediate shaft constructed of universal joints 107, 110 and shaft members 108, 109.
It is generally known that, in the thus constructed steering apparatus, the shaft member of the steering intermediate shaft is constructed as a mechanism for absorbing, when collided, an impact energy and a movement and a deformation of the steering apparatus by an entire length shrinking upon receiving an impact in order to protect a driver from a damage just when a vehicle collision happens. What is widely used as this mechanism is that the steering intermediate shaft is structured so that an outer tube member and a solid inner shaft member are combined with each other by a spline or serration engagement enough to permit a relative movement in the axial direction therebetween. To give an explanation with reference to FIG. 1, an upper portion 108 of the shaft member of the intermediate shaft is formed as a tube member the inner surface of which is formed with a serration or a spline, while a lower portion 109 is formed as a solid inner shaft member the outer surface of which is formed with the serration or the spline. Then, this solid inner shaft member is inserted into the tube member, thus making the serration or spline engagement.
It is, as described above, useful also when assembling the steering apparatus to give a degree of freedom of the axial movement to the intermediate shaft. When the vehicle is assembled, normally the steering gear portion for changing a direction of the wheel is at first fixed to a car body chassis, while the intermediate shaft portion is fixed to the body side integrally with the steering column, and these components are assembled afterward. This is a procedure of how the apparatus is assembled. To describe it referring to FIG. 1, the shaft 111 of the steering gear apparatus exists on the side of the chassis, and the portions higher than the universal joint 110 exist on the body side. When in the assembling process, it follows that the shaft 111 is joined to the joint 110. It is therefore necessary and useful for the intermediate shaft including the inner shaft member and the outer tube member to have the degree of freedom of the movement (expansion and contraction) in the axial direction.
Further, as disclosed in Japanese Utility Model Post-Exam Publication No. 63-17862, there has hitherto been known a structure for reducing backlash in the engaging portions between one end of the outer tube member and one end of the inner shaft member.
FIG. 2 shows an expandable/contractible intermediate shaft disclosed in this Japanese publication. An inner peripheral surface of an outer tube member P1 is formed with a female spline P2, and an outer peripheral surface of a solid inner shaft member P3 is formed with a male spline P4 engaging with the female spline P2. Further, slits P5, P5 each opening at one edge of the outer tube member P1 are formed in a plurality of positions in a circumferential direction of one end of the outer tube member P1. Moreover, a ring-like fastening member P6 is externally fitted to one end of the outer tube member P1. This ring-like fastening member P6 elastically fastens an inner peripheral surface of one end of the outer tube member P1 onto the outer peripheral surface of the solid inner shaft member in a state where the inner shaft member P3 is inserted into the outer tube member P1. Then, a backlash in engaging portions between the female and male splines P2, P4 is thereby prevented irrespective of a minute gap existing between the female spline P2 and the male spline P4.
While such an assembly has allowed for reduced lash and noise in the steering shaft assembly, such an assembly requires significant modification of the intermediate shaft outer tube. Such modification is generally costly and impractical based on the numerous variations of outer tubes utilized in various vehicles. A modification to the production process for each of these various tubes would be required. Furthermore, the ring-like fastening member P6 is a loose member which may be lost during production. Additionally, the ring-like fastening member P6 requires an extra assembly step, which if forgotten by assembly personnel will leave the ring-like fastening member to move and shake during vehicle operation and will further fail to provide the desired damping.